In order to maintain the necessary cryogenic temperature, superconducting magnets such as those used in MRI scanners must be suspended inside a vacuum vessel. In order to fully constrain the suspended vessel under the loads encountered during operation and transportation, conventional designs employ multiple suspension elements. These elements are complicated and require multiple attachments to the vessels. The result is an expensive suspension system which is time consuming to assemble and not well suited to volume production conditions.
FIG. 1 illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system. A number of coils of superconducting wire are wound onto a former 1. The resulting assembly is housed inside a cryogen vessel 2 which is at least partly filled with a liquid cryogen 2a at its boiling point. The coils are thereby held at a temperature below their critical point.
Also illustrated in FIG. 1 are an outer vacuum container 4 and thermal shield 3. As is well known, these serve to thermally isolate the cryogen vessel 2, typically containing a liquid cryogen 2a, from the surrounding atmosphere. Solid insulation 5 may be placed inside the space between the outer vacuum container 4 and the thermal shield 3. A central bore 4a is provided, of a certain dimension to allow access for a patient or other subject to be imaged.
Conventionally, a number of supporting elements 7 are connected between the cryogen vessel 2 and the outer vacuum container 4 to bear the weight of the cryogen vessel. These may be tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. The elements should have a very low thermal conductivity, in the case that a cryogen vessel is supported. This is important in order to minimise heat influx from the outer vacuum container 4, which is typically at ambient temperature, to the cryogen vessel 2. The suspension elements typically pass through holes in the thermal shield 3 and the insulation 5. Similar, or alternative, suspension arrangements may be provided to retain the thermal shield 3.
The suspension elements must be of the minimum cross sectional area and maximum length in order to minimise the heat flow into the magnet. In conventional designs such as shown in FIG. 1, multiple tension-only elements or combinations of tension and compression elements are employed. These may be made from high strength steel or advanced composite materials using glass, carbon or other suitable load-bearing fibres. Typically a minimum of eight elements are used for each vessel, giving a total of sixteen elements to support a typical magnet and radiation shield system. Such a system will typically contain hundreds of individual parts, which must be individually assembled. Since MRI systems are commonly transported fully assembled and filled with cryogen, the suspension must be capable of resisting the high loads associated with transportation and handling. The suspended vessels must be accurately constrained at all times, and when conventional elements are used this requires that the elements are pre-loaded to ensure than they remain tight under all the foreseeable design loads. Due to the need to achieve the smallest overall system dimensions, space for the suspension is limited, and access is further restricted by the insulation blankets that must be used. Each suspension element passes through holes in the thermal shield 3 and the insulation 5. These holes must be sealed during assembly, once the suspension elements are in place, to maintain effective thermal insulation and shielding. As a result, the assembly procedure is complicated and time consuming, and not well suited to high volume manufacture.
EP 1001438 describes a tube suspension arrangement for use in a superconducting magnet, wherein the cryogenic vessel may move relative to the thermal shield and the OVC during cool down from ambient temperature to cryogenic temperature.
U.S. Pat. No. 6,358,583 describes an arrangement wherein a magnet structure is supported on a thermal shield by a first tubular support, the thermal shield being supported on the OVC by a second tubular support.
U.S. Pat. No. 5,530,413 describes a cooled solenoid magnet in a cryostat arrangement, wherein the magnet and thermal shield are supported by tubular support structures coaxial with the solenoidal magnet.